The present invention relates generally to pressure rolls for pressure treating sheets, and more particularly to a roll which is suitable for use with an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules. The pressure roll may be used within apparatus for rupturing the microcapsules on such imaging sheets to produce an image.
U.S. Pat. Nos. 4,440,846 and 4,399,209, which are assigned to the same assignee as this invention, and which are hereby incorporated by reference, describe an imaging system wherein a photosensitive layer, comprising microcapsules containing a photosensitive composition in the internal phase, is imagewise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and imagewise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet developing processing solutions to produce the image. An image forming chromogenic material, such as a substantially colorless color form, is typically associated with the microcapsules. When the microcapsules rupture, the color former imagewise reacts with the developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing imagewise exposed imaging sheets through the nip between a pair of parallel calender rolls.
The media may exist in either single sheet or two sheet versions. In the former case, the microcapsules and developer composition are both coated onto a single substrate layer. In the later case, the microcapsules are carried on a first substrate layer referred to as a donor sheet. The developer composition is coated onto a second, separate substrate layer referred to as a receiver sheet. The donor sheet is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheet. In either case, the image is developed by pressure, with the finished image being formed in the sheet carrying the developer layer.
While heavy pressure is not required to rupture the microcapsules, high pressure is normally used to develop the imaging sheets. To normalize surface irregularities in the imaging sheets, substantial compression of the paper must be achieved. Otherwise, if the rupturing force is not uniformly distributed, the imaging sheets develop unevenly and the tonal characteristics of the resulting images are degraded. Thus, typical developing pressures of 6,000 to 8,000 psi are required.
Unless very large diameter rolls are used for development, it is difficult to achieve uniform high pressure across the width of a nip between a pair of simple developer rolls. The rolls can be loaded only at their ends, and the pressure exerted upon a sheet passing between the nip of the rolls will be greater towards the roll ends and at a minimum value at the nip center point.
To overcome these difficulties in the art of fixing fusible toner images, it is known to use a pair of rolls biased toward each other and disposed such that the axis of one roll extends at an angle relative to the axis of the other. This skewed arrangement tends to minimize irregularities of the nip between the rolls and to ensure uniform pressure along the nip despite bending of the rolls. However, when the skew angle is sufficiently large to overcome roll bending, creases or wrinkles may be formed in a paper sheet passing between the nip.
Yet another approach is shown in U.S. Pat. No. 4,343,234, in which a three-roll system includes a pair of pinch rolls disposed with a slight skew angle therebetween, and a third back-up roll disposed in a crossing contact relative to one of the pinch rolls. The back-up roll is biased resiliently upwards towards the second roll of the pair of pinch rolls by a pressure mechanism such that the second roll is butted against the first roll under pressure to form a nip through which paper sheets pass. The bending of the second roll can be adjusted by the cross angle of the back-up roll relative to the second roll such that both the second roll and the back-up roll may have a smaller diameter than the first roll. Even in this case, however, the skew angle between the nip rolls must still be relatively small to avoid wrinkling of the sheets.
A particular inherent disadvantage of such prior art pressure systems is that the pressure across the nip is uniform only at one designed pressure, and variations in the thickness of the material passing through the nip or variations in the pressure applied to the nip causes a departure from a condition of pressure linearity across the nip.
An alternate approach to the problem of bending rolls can be seen in U.S. Pat. No. 4,581,797. A pair of pressure rolls includes one conventional roll, and a second roll formed from a sleeve rotatably mounted on a shaft. A plurality of support elements extend between the roll shaft and the inner surface of the roll sleeve. Fluid under pressure is applied to each of the support elements to compensate for the bending of the support shaft and to produce a linear nip along which uniform pressure is applied. Each of the support elements can be individually controlled to permit relatively accurate adjustment. However, the disclosed device requires a relatively complex mechanical and fluid distribution structure, and is not particularly well suited for use in small diameter pressure rolls such as might be used in an apparatus for pressure development of paper sheets.
What is needed, therefore, is an apparatus which is relatively light in weight, can be easily constructed to be relatively small of size, and which is capable of providing a substantially uniform nip pressure throughout a design width of the roll over varying thicknesses of sheet material and over varying overall loadings on the rolls. Such an apparatus should be inherently capable of applying a uniform pressure across a nip, over long periods of time, and with a minimum of maintenance requirements.